Glucokinase (GK) plays an essential role in glucose homeostasis. In humans, mutations in this enzyme cause both maturity onset diabetes of youth, type 2 (MODY-2) and persistant hyperinsulinemic hypoglycemia of infancy (PHHI). Although GK gene mutations are one of only a few genetically-defined causes of diabetes, we do not yet fully understood how mutations in this gene actually alter the plasma glucose concentration. This is a proposal to make use of novel lines transgenic and gene- altered mice to learn more about the cell-specific roles of glucokinase (GK) in glucose homeostasis and about adaptive responses that occur in response to acute and chronic alternations in the plasma glucose concentration. Aim 1 will define the tissue-specific functions of GK in glucose homeostasis by selectively removing it from different cell types using a Cre/LoxP strategy. Aim 2 will develop mice with inducible defects in either hepatic glucose utilization and/or beta cell insulin secretion, and characterize the adaptations that occur in response to both an acute loss of either hepatic or islet GK. Together, these studies will provide fundamental new insights into the molecular pathogenesis MODY-2 as well adaptations that occur in response to hyperglycemia. Thus, these studies are also relevant to the pathogenesis and treatment of Type 2 diabetes mellitus.